In known LNG carriers, LNG in a cargo hold is vaporized to generate boil-off gas (BOG). Since it is very expensive to re-condense this BOG, the BOG generated in the cargo hold of the LNG carrier is used with a steam turbine, which is a main engine. The steam turbine makes use of steam, and thus requires a condenser, etc. as an apparatus for cooling this steam. The system for cooling this condenser employs a scoop cooling system or a pump cooling system. In this system, an inlet through which sea water is introduced (cooling water inlet) and an outlet (cooling water outlet) are installed in the surface of the hull. In the scoop cooling system, the outlet protrudes in the shape of a lip. In the pump cooling system, no lip is provided for the outlet.
This cooling water outlet is located in the surface of the hull on the port side of an engine room portion of the LNG carrier, that is, on the left-hand side of the hull towards the stern when viewed from the stern of the hull. When the cooling water is discharged through the cooling water outlet, the flow field behind the cooling water outlet is turbulent. This flow-field turbulence, occurring behind the cooling water outlet, speeds up the axial flow of the sea water introduced toward the upper part of a propeller, and changes the distribution of a wake flow so that the distribution of the wake flow is asymmetrical around the central axis of the hull. In the case in which the propeller is situated behind a model ship or an actual ship, the propeller is operated in the fluid, which becomes turbulent while passing along the hull. In general, the fluid around the stern has a slowing effect on the ship. This slowing flow is called wake flow.
This irregular distribution of wake flow makes the load applied to the blades non-uniform when the propeller is operated, thereby giving periodical force and moment to the hull, and furthermore leading to vibration of the hull. In comparison with the case in which no water is discharged, such wake flow increases cavitation of the propeller, which results in approximate doubling of the variable pressure of the hull. The term “cavitation” (formation of cavities) refers to the phenomenon in which water is converted from a liquid state to a gas state when ambient pressure becomes lower than vapor pressure under constant pressure.
This increase in the variable pressure of the hull not only increases the vibration of the hull but also reduces propulsive performance of the ship.
As described above, the cooling water causes the flow-field turbulence at the stern, and this flow-field turbulence changes the distribution of the wake flow so that the distribution of the wake flow is asymmetrical around the central axis of the hull. For this reason, no mechanism of decreasing the vibration of the hull has been known to date.